fix(config): Make default values for executables user-visible

[cbandera]

This change makes the default settings for the executable paths visible from the VSCode GUI and simplifies the code by moving the defaults to package.json.

Closes #88

[cameron-martin]

After this change if you set the settings to the empty string it fails to spawn bazel or buildifier. Since the empty string is never a valid executable, maybe we should keep the behaviour where we set a default value when the setting is the empty string?